A known active matrix substrate for use in a liquid crystal display device, etc., includes a plurality of independently driven pixel electrodes arranged in a matrix form, and switching elements such as TFTs (Thin Film Transistors), etc., provided for respective pixel electrodes. In the liquid crystal display device adopting such active matrix substrate, an image is displayed by sequentially selecting the switching elements by scanning lines and reading potentials of signal lines into the pixel electrodes via the switching elements.
The foregoing active matrix substrate can be used for an image sensor. Examples of known image sensors adopting the active matrix substrate include: an image sensor including a conversion layer formed on an upper layer of the active matrix substrate, for directly converting incident electromagnetic wave such as a light beam, an X-ray, etc., into electric charge, wherein the electric charge generated from the conversion layer is stored in pixel capacitance at high voltage, and the electric charge is read out sequentially from the pixel capacitance. For example, Japanese Unexamined Patent Publication No. 212458/1992 (Tokukaihei 4-212458) published on Aug. 4, 1992, discloses an image sensor of the above type wherein electric charge as generated by the conversion layer is stored in auxiliary capacitance, and data (potential data) are stored in respective pixels in the form of electric charge according to the characteristics of an object. As in the case of the aforementioned liquid crystal display device, by sequentially scanning the scanning lines, for example, the data stored in a pixel selected by a scanning line is read out and transmitted via a switching element to a signal line, and an image projected to the image sensor is read out from a circuit such as an operation amplifier provided on the other end of the signal line.
The active matrix substrate, which is a precursor to the sensor in the foregoing example can be manufactured at low costs without requiring any additional facilities, because the manufacturing process for liquid crystal display devices can be used for the manufacturing process of image sensors only by adjusting the dimensions of the pixel capacitance and the time constants of the switching elements to be optimal for image sensors.
FIG. 6 is a cross-sectional view illustrating a schematic structure of a known example of the basic image sensor adopting an active matrix substrate. The structure illustrated in FIG. 6 is disclosed in AM-LCD' 99 “Real-time Imaging Flat Panel X-Ray Detector” by M. Ikeda, et al. As illustrated in FIG. 6, the active matrix substrate of this sensor is prepared by forming a switching element 51 on a transparent insulating substrate 55, and further vapor-depositing thereon a conversion layer 66 and a metal layer 67 in this order. The switching element 51 is prepared by forming on the transparent insulating substrate 55, a gate electrode 56, an auxiliary capacitance electrode (not shown), a gate insulating film 57, a semiconductor layer 58, an n+-Si layer 59 to be patterned into a drain electrode, a metal layer 60 and a transparent electrically conductive film 61 to be patterned into a source signal line, and a protective film 62 in this order, thereby forming a substrate of the image sensor. The conversion layer 66 is provided for converting an X-ray into electric charge. The metal layer 67 is patterned into an electrode for use in applying a voltage to the conversion layer 66. In the foregoing structure, the transparent electrically conductive film 61 is patterned into the pixel electrodes for storing the electric charge as converted in the conversion layer 66.
In the image sensor, the electric charge is read out from respective pixel electrodes in contrast to the liquid crystal display device in which electric charge is applied to the pixel electrodes. Therefore, if a normal readout operation of a predetermined cycle is not performed due to any failure, or a trouble in signal readout program, unexpectedly large electric charge may be stored in the pixel electrode, and the resulting high voltage may cause a damage on the active matrix substrate. The foregoing problem is discussed in “Characteristics of dual-gate thin film transistors for applications in digital radiology” (NRC'96) in “Can. I. Phys. (Suppl)74 published in 1996, in which the following structure has been proposed as a solution to the problem. That is, a pixel electrode is extended over a switching element, so that the pixel electrode can be functioned as one of the gate electrodes of a dual-gate transistor, and at or above a predetermined threshold voltage, the transistor is switched on, and excessive electric charge is released.
The structure of an image sensor which is particularly effective in preventing the foregoing problem will be explained in reference to FIG. 7. As illustrated in FIG. 7, the image sensor has a so-called “mushroom structure” wherein pixel electrodes 72 and source lines 71 are formed in different layers so as to be insulated by an insulating layer 73 formed in between, so that the entire channel region W of a transistor 74 is covered with the corresponding pixel electrode 72. In FIG. 7, the reference numerals 75, 76, 77, 78 and 79 indicate a gate electrode, a drain electrode, an auxiliary capacitance, a conversion layer and a semiconductor layer respectively.
The foregoing structure of Waechter, et al, illustrated in FIG. 7 is effective for the high voltage protection in the pixel electrodes 72. As to the size of the pixel electrodes 72, however, significant improvement from the aforementioned active matrix substrate illustrated in FIG. 6 cannot be expected. It is generally known that the larger is the area occupied by the pixel electrodes 72, the more efficiently, the electric charge generated from the conversion layer 78 can be collected in the pixel electrodes 72. In the generally used active matrix substrate, however, there is a limit for an increase in size of each pixel electrode as pixel electrodes are arranged in a plane with certain intervals from source bus lines.
In the foregoing structure of FIG. 7 wherein the insulating film 73 is formed between the source line 71 and the pixel electrodes 72, the pixel electrodes 72 can be formed over the source lines 71 while maintaining the insulation between them. In this state, the electrostatic capacitance is generated between the pixel electrodes 72 and the source lines 71, and an overall capacitance of the source lines 71 when seen from the side of the signal readout circuit increases, and a noise of the readout signal is increased, resulting in lower signal to noise (S/N) ratio. For the foregoing reasons, the structure of FIG. 7 would not offer any significant improvement in size of the pixel electrodes 72 from the conventional active matrix substrate.
In the X-ray image sensor, generally a large pixel capacitance is ensured. For this reason, the capacitance between the pixel electrode 72 and the source line 71 becomes a load capacitance to the source line 71 directly. On the other hand, internal noise generated in the signal readout amplifier is amplified by a gain in proportion to the ratio of the capacitance of the source line 71 to the feedback capacitance. It is therefore effective to reduce the capacitance of the source line 71 for a reduction in internal noise.
Further, an increase in capacitance of the source line 71 may cause variations in potential of the source line 71 corresponding to the capacitance CsD (per pixel) between the pixel electrode 72 and the source line 71 with changes in pixel potential corresponding to the part of the image irradiated with an X-ray. For example, when reading out signals via the source line 71 with a selection of certain scanning line, the electric charge is kept being stored in other pixel electrodes, while the electric charge in positive polarity and in proportion to the capacitance CsD are being stored in the source line 71. The amount of the electric charge to be stored in the pixel electrodes and the source line 71 differ depending on the image on an entire screen, thereby presenting a problem that a so-called crosstalk is generated when reading out signals as being affected by pixel electrodes aligned in direction parallel to the source line 71.
In order to reduce the capacitance of the source line 71, for example, an image sensor adopting an interlayer insulating film made of photosensitive resin has been proposed, for example, in “Similarities between TFT Arrays for Direct-Conversion X-Ray Sensors and High-Aperture AMLCDs” (SID 98 DIGEST) by W.den Boer, et al, published in 1998.
However, W.den Boer, et al does not refer to the dual-gate structure.